Methods for improved deconvolution of seismic or similar data

ABSTRACT

Methods for preparing seismic data to be deconvolved comprise the introduction of a time variable correction based on velocity in the reverberating media and dip at the reverberating boundaries. This invention corrects for angularity in the reverberation path introduced by the detector being remote from the source and for the dips of the boundaries by making the undesired reverberations uniformly repetitive. Either normal movement (NMO) or composite corrections for NMO and dip are used depending on the extent of dip of the reflection boundaries, or the degree to which the refraction boundaries are not parallel, to make the reverberations repetitive in time. The modified data is then deconvolved to remove the repetitive noise and retains the desired signals which are non-repetitive. After deconvolution, a second time variable correction, representing the difference between the NMO correction required for the desired primary reflections and the first time variable correction, is applied. The data is then stacked using horizontal stacking techniques or further processed to provide any other desired form of presentation.

United States V Patent Q y 145 Dec. 5, 1972 METHODS FOR IMPROVEDDECONVOLUTION 0F SEISMIC OR SIMILAR DATA [72] Inventor: Roy G. Quay, SanAntonio, Tex.

[73] Assignee: Petty Geophysical Engineering Company, San Antonio, Tex.

[22] Filed: Feb. 26, 1970 [2l] Appl. No.: 14,606

1521 us. (:1 ..340/1s.s cc, 340/155 MC, 340/155 T( 51 1111. c1. ..G0lvH28 [58 1 Field ofSearch ..340/15.5 cc, 15.5 TC, 340/155 MC [5 6]References Cited ummap STATES PATENTS 3,492,469 1/1970 SiIVelman..340/15.5 MC 3,409,871 11/1968 Heffring 340/155 cc PrimaryExaminerBenjamin A. Borchelt Assistant Examiner-N. MoskowitzAtz0rneyArn0ld, White & Durkee, Frank S. Vaden,

I [11, Paul VanSlyke and Tom Arnold SOUIZC E [57] ABSTRACT Methods forpreparing seismic data to be deconvolved comprise the introduction of atime variable correction based on velocity in the reverberating mediaand dip at the reverberating boundaries. This invention corrects forangularity in the reverberation path introduced by the detector beingremote from the source and for the dips of the boundaries by making theundesired reverberations uniformly repetitive. Either normal movement(NMO) or composite corrections for NMO and dip are used depending on theextent of dip of the reflection boundaries, or the degree to which therefraction boundaries are not parallel, to make the reverberationsrepetitive in time. The modified data is then deconvolved to remove therepetitive noise and retains the desired signals which arenon-repetitive. After deconvolution, a second time variable correction,representing the difference between the NMO correction required for thedesired primary reflections and the first time variable correction, isapplied. The data is then stacked using horizontal stacking techniquesor further processed to provide any other desired form of presentation.

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WM, 636,?144/5 flaw JTTO/Q/VfYS METHODS FOR IMPROVED DECONVOLUTION OFSEISMIC OR SIIVIILAR DATA FIELD OF THE INVENTION tions or refractions inmany areas. Conventional decon- 1 volution has been adequate for datafrom detectors near the source if the surface and subsurface boundariesare parallel. If the detectors are remote from the source or if theboundaries are not parallel, conventional deconvolution has. not beenadequate I and frequently deteriorates the data. The methods disclosedherein are applicable to Sonar techniques or seismic processingtechniques in general.

DESCRIPTION OF THE PRIOR ART In reflection seismic exploration, aseismic source is excited at or near the surface, seismic energy istransmitted'down to a series of reflecting boundaries, and is reflectedto a plurality of spaced detectors at or near the surface. In additionto these desired primary reflections, seismic energy is repeatedlyreflected among several of these boundaries or between the surface and amajor subsurface boundary. These repeated reflections are calledmultiple reflections, or if they are closely spaced in time, may becalled reverberation, ringing, singing, and other similar terms.

The output signals from the plurality of spaced detectors are fed into aplurality of amplifiers and recorded. The signals are usually convertedfrom an analog form and recorded on magnetic tape in a digital format.In order to reduce the reverberation or multiple reflections, theprocessing may include deconvolution such as described by Peacock andTreitel in the Apr., 1969 issue of Geophysics, pages 155 thru 169. Theoutput data may then be corrected for NMO based on the velocity to thedesired primary reflections and converted to a visual display.Additional steps may be taken after correction for NMO such as stackingaccording to US. Pat. No. 2,732,906, issued Jan. 31, 1956, to Mayne. Inthis latter case, the deconvolution may be applied after a series ofreflections from a common reflection point have been stacked.

A predictive decomposition of time series with application of the timeseries to seismic exploration by Enders A. Robinson was published inGeophysics, June, 1967, pages 418 thru 484. This paper includes amathematical description of the process and an extensive bibliography.Marr and Zagst apply deconvolution to horizontal data stackingtechniques of Mayne in Geophysics, Apr., 1967, pages 207 thru 224.

SUMMARY OF THE INVENTION The method in accordance with this invention isespecially suited to improve the deconvolution processes previouslycited. The source is usually at an appreciable distance from most of thedetectors. Even if both the surface and the reflecting horizon arehorizontal, the multiple reflections or reverberations will not beuniformly spaced in time due to the angularity of the path. Sincereflecting horizons and the surface of the ground may not be horizontal,additional time variations are present in multiple reflections orreverberations. Even if the detector is at the source, a variation inthe thickness of the media causing the reverberation will shift the timeinterval between the reverberation and make the reverberationnon-repetitive. In marine work, changes in the characteristics of thebottom, such as from a hard to a soft surface, may

0 further shift the repetition. My invention comprises the method ofintroducing the appropriate time variable corrections to make the timeinterval constant between repeated multiple reflections orreverberations, storing the information as to the time variablecorrections, and

after deConvolving the modified data, introducing a second group of timevariable corrections generated from the difference between the desiredNMO correction for primary reflections and the first time variablecorrection to align the primary reflections (preferably from the samecommon reflection point) before stacking or making a visual display. Inthis manner, the multiple reflections or reverberations will have thesame time interval between repeated events and are removed bydeconvolution.

When the autocorrelation is made in the deconvolution process, therewill be a large peak response for this repetitive time interval ratherthan a series of small peaks for the series of time intervals obtainedusing the prior art. The automatic design and application of the filterfor the large peak can then proceed as is known in the prior art. Thepreferred form of the second time variable correction would be the NMObased on the velocity of the desired primary reflections as is normallyused in the prior art, minus the first time variable correction selectedaccording to my invention. The second time variable correction willusually be negative which means that the time shift is in the oppositedirection than the first time variable correction. This is because NMOcorrections at any specified time are larger for low velocities than forhigh velocities.

In summary, the method requires the insertion of additional processingsteps before and after deconvolution to overcome complications inducedby dip, nonparallel boundaries, and/or remote detectors.

BRIEF DESCRIPTION OF THE DRAWINGS The above-recited, as well as other,advantages and objects of the invention will become apparent from thedrawings and specifications.

FIG. 1 illustrates the longer path length per reverberation for alow-order multiple reflection and the shorter path length perreverberation for a higher-order multiple reflection;

FIG. 2 illustrates the complications introduced where the reflectinghorizons have appreciable dip;

FIG. 3 illustrates a refinement of FIG. 2, with both a detector and asource located at the same surface location;

FIG. 4 illustrates the steps of the invention; and

FIG. 5a shows apparatus for applying and removing corrections from theseismic data; FIGS. 5a-5d illustrate apparatus for determining theinverse filter and deconvolving the data.

DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 illustrates multiplereflections and reverberations from a horizontal reflector 31. Seismicsource is located on or near the surface 30 of the water during marineoperation, or in the case of land operation source 10 is located on ornear the surface of the ground. Seismic energy from source 10 travelsdown to reflection point 12 on reflector 31 and is reflected up todetector 11 on or near surface 30 as a primary'reflection. The seismicenergy also follows many other paths such as the multiple reflectionfrom source 10 to reflection point 13 on reflector 31, up to point 14 onthe surface ar at the base of the weathered layer, down to reflectionpoint 15 on reflector 31, and up to detector 11 on or near surface 30.If the travel time is sufficiently different than the travel time ofother reflections from this same reflector, this path would beconsidered a multiple reflection. If the reflections overlap, this maybe called a reverberation, ringing, singing, etc. Additional multiplereflections or reverberations exist, such as the path from source 10 toreflection point 17 to point 18, to reflection point 12, to point 19, toreflection point 20, to detector 11. Actual measurements of path lengthcould be made to show the path 10-13-14-15-11- is longer per one-waytravel path than path 10-17-18-12-19-20-1 1-. However, it is obviousthat the path from source 10 to reflection point 13 is longer than thepath from source 10 to reflection point 17, which is longer than thepath from source 10 to reflection point 22, and so on, with the finallimit of a vertical path from source 10 to reflection point 25. Anotherway to consider these paths is to use the path images. For example, thepath from point 13 to point 16 to point 15 is the image of the path frompoint 13 to point 14 to point 15 since all corresponding distances arethe same but the triangle formed by the paths is inverted. The imagepath from point 17 to point 21 to point 20, is the same length as thepath from point 17 to point 18 to point 12 to point 19 to point 20;however, the image appears as a primary reflection from a deeper horizonwith the same velocity as in the medium 32, which is between surface andreflector 31. It can be readily observed from these images that thetravel path is progressively longer for the higher order multiplereflections than for a primary reflection. This progressively longerone-way travel path and corresponding travel time seriously deterioratesthe effectiveness of deconvolution or decomposition as will be describedlater. It should be pointed out that if a detector could be used at thesource, the successive multiple reflections between a horizontal surfaceand a horizontal reflector would have the same time interval.Unfortunately, it is seldom possible to use a detector at the sourcebecause of noise from the source. Also, economic reasons dictate that aplurality of detectors or detector groups such as 24, 36 or 48 in numberbe used to record data from each source, and these detectors should havesuccessively greater source-to-detector distances. The horizontalstacking technique of Mayne is the most effective method to attenuatemultiple reflections. Since the longest source-to-detector distance inaccordance with that technique is usually more than l mile and often 2miles, the non-uniformity of time intervals is very large.

FIG. 2 illustrates a further complicating feature, the effects ofreflectors having appreciable dip. Seismic energy from source 50 istransmitted as a primary reflection down to reflection point 52 onreflector 71 and up to a detector 51 on or near the surface 70. Amultiple reflection is indicated from source 50, down to reflectionpoint 53, up to point 54, down to reflection point 55, and then back todetector 51. An image is dotted from point 53 down. to point 56 onreflector image 66 and up to point 55 similar to the image process inFIG. 1. The image of the successive multiple reflections are shown inrelation to points 61 and 63. These multiple reflections differ fromthat obtained from horizontal reflector 31 in FIG. 1. The successivepaths in a multiple reflection have different path lengths, for examplefrom source 50 down to reflection point 53 and back up to point 54 islonger than the path from point 54 down to point 55 and back up todetector 51. Hence in this case, the entire path length must beconsidered so the use of image reflectors 66, 68, and 69 are veryuseful.

The dip of image reflector 66 is twice that of reflector 71, the dip ofimage reflector 68 is three times that of reflector 71, and the dip ofimage reflector 69 is four times that of reflector 71. The total pathfrom source 50 to point 56 to detector 51 can be determined by swingingan arc of a circle from source 50 to point 72 using point 56 as thecenter and measuring about 6.5 units on this scale from point 72 todetector 51. The total path from source 50 to point 61 to detector 51 isthe same as point 73 to detector 51 or about 7.95 units. The total pathfrom source 50 to point 63 to detector 51 is the same as point 74 todetector 51 or about 9.05 units. The difference between consecutivepaths is l .45 (7.95-6.50) units and 1.10 (9.05-7.95) units. While thisis a progressing decrease in the path interval, this may not be the casefor higher order multiple reflections. Note that reflection point 64 ison the right side of point 66 than points 20 and 55, are on the leftside of point 66. The line from detector 51 to point 66 is perpendicularto reflector 71. Reflection points for higher order multiple reflectionswill be farther to the right and this distance will increase, resultingin longer intervals between high order multiple reflections. Thisillustrates that the reflections obtained from reflectors of steep clipwill vary from the reflections obtained from a horizontal reflector. Thehigh order multiple reflections, or reverberation, are particularlysevere in most marine work. There is usually a good reflector near thewater bottom, thus resulting in little attenuation of seismicfrequencies in the water. In such a situation, hundreds ofreverberations may occur on a conventional seismic record. A morecomplete discussion of multiple reflections and reverberation isdescribed by V. A. Olhovich in'Geophysics, December 1964, pages I ,015thru 1,030. This paper also includes other factors such as the velocitygradation with respect to depth which produces curved ray paths ratherthan the straight line paths illustrated in FIGS. 1 and 2. Curved raypaths produce further changes in the time interval of multiplereflections or reverberations.

FIG. 3 illustrates that even with the detector located at the source, awedge shaped reverberating media would result in non-repetitive multiplereflections or reverberation. Combined source and detector 75 is locatedon or near surface 76. Energy from source-detector 75 is transmitted toreflector 77 and is reflected at reflection point 78 back tosource-detector 75. Multiple reflections do not follow this same path aswould be the case in FIG. 1. Energy from reflection point 78 tosource-detector 75 could follow path 79 and not return to thesource-detector 75. A multiple reflection would have the path fromsource-detector 75 to reflection point 81 on reflector 77, up to point82 on or near surface 76, then be reflected back this same path down toreflection point 81, and up to source-detector 75. If the dip ofreflector 77 is very steep, only a limited numberof reverberations canexist theoretically, but in actual cases many reverberations do exist.High order multiples or reverberations would have paths such as fromsource-detector 75 to reflection point 83, up to point 84, down toreflection point 85, up to point 86 and return by the same path (down toreflection point 85, up to point 84, down to reflection point 83, and upto source-detector 75).

DECONVOLUTION OF DATA FROM DETECTORS REMOTE FROM THE SOURCE Thefollowing brief description of deconvolution, or decomposition, willindicate the problems encountered when deconvolving data from detectorswhich are remote from the source.

The terms deconvolution and decomposition define techniques forremoving, by inverse filtering techniques, something that was convolvedwith the data upon recording. The unwanted data sought to be removed canbe such things as field instrument responses, water ringing effects,multiple reflection interferences, frequency variations from record torecord, seismometer depth effects, peaked" amplitude responses, etc.

The first basic assumption which is made to justify deconvolution, isthat all valid primary events are spaced at random times on theseismogram, i.e., primary reflections will not be periodic in nature.Secondly, it is assumed that multiple reflections and reverberations areperiodic in nature. Finally, the assertion is made that a balanced orwhite frequency spectrum, and not a peaked" spectrum, is necessary forvalid data character enhancement and resolution.

The tool must be found that will yield the necessary information aboutthe periodicity of the unwanted data, and the frequency content of theunwanted data. This tool is the auto correlation function. From thisfunction, the amplitude response and the phase response of the periodicevents are determinable. With the phase and amplitude response of theunwanted signal defined, an inverse filter is designed, using theinverse amplitude and phase characteristics of the unwanted data, whichwhen convolved with the trace will remove or greatly suppress, theunwanted signal.

To be more specific, a filter is designed with the multiplicativeinverse of the amplitude response of the unwanted data, and the additiveinverse of its phase response. Due to the fact the convolution processmultiplies the amplitude response and adds the phase response, theunwanted data is removed by application of this inverse operator.

Now it is apparent that peaked amplitude responses due to bandlimitedfield recording, seismometer depths, ringing, etc., can be removed withinverse filtering, or deconvolution. It is also apparent that events ofa periodic nature are removed due to the inverse phase response of theoperator.

The second assumption is that multiple reflections and reverberationsare periodic in time. This assumption does not apply to data fromdetectors remote from the source. This assumption does apply, however,to data recorded by a detector at the source if the reflector ishorizontal. The crux of this invention is the method of, and means for,converting these multiple reflections or reverberations into repetitiveevents prior to deconvolution, then after deconvolution, es sentiallyremoving the correction previously applied. The latter step ispreferably combined with the introduction of NMO for the desired deepprimary reflections to reduce computer time.

If the reflector causing the multiple reflections or reverberations issubstantially horizontal, a simple method can be applied with minimumeffort. FIG. 1 illustrates that the image reflections can duplicate thetravel paths of the multiple reflections. For example, the path fromsource 10 to reflection point 13, thence to point 14, thence to point15, thence to detector 1 l is the same length as the path from source 10to image point 16 and then to detector 11. The average velocity down toreflector image 27 would effectively be the same as in medium 32 down toreflector 31. Similar conditions apply to the other image paths 21 and23. Hence this reverberation in medium 32 can be represented as a seriesof primary reflections from a series of equally spaced reflector images.The velocity in the entire section would be the same as in medium 32. Inthe case of marine work, this would be water velocity when thereverberation is in the water. Paths 10-16-1 1-, 10-21-11-, and l023-11can be converted to paths 1033-10, l034-l0, and 10-35-10- respectivelyby application of normal moveout, NMO, using a8 constant velocity of themedium 32. Then the reverberations on the corrected data would haveconstant time intervals because the distant intervals 25 thru 33, 33 to34, and 34 to 35 are equal and the velocity is a constant. This is asimple process and can readily be applied by a properly programmeddigital computer.

FIG. 4 is a flow chart of the steps for carrying out the invention. Thefield data is usually recorded in digital format on magnetic tape. Thetape is sent to the processing center and placed on a digital tapetransport where the data is read into a digital computer at 100. Ifnecessary, the data is reformatted and edited at 101. Reformatting andediting may be necessary, for example, to conform the data to the formatof the computer and the program utilized for operating it. Staticcorrections due to elevation, weathering, and similar constant timeshifts are introduced at 102. If the reflector is essentiallyhorizontal, as is the case in many areas, an NMO is applied to the datain 102 using the average velocity in the media causing thereverberation. This NMO can be computed using travel paths such as shownin FIG. 1 and determining the difference in travel time between thediagonal and vertical paths. This difference is the NMO correction. Aconventional deconvolution or decomposition is applied at 103. Afterdeconvolution, the NMO applied to the data at 102 is removed therefromby application of the correction in the opposite sense or direction.Then normal methods of seismic processing, such as horizontal stacking,are applied at 105 and any desired display of the processed data isprepared at 106. It is usually more efficient to remove the correctionapplied at 102 simultaneously with the removal of NMO for the primaryreflections. To accomplish this, a time variant correction is applied tothe deconvolved data at 104. This preferred correction is equal to theNMO for deep primary reflections minus the NMO for multiple reflectionsapplied at 102.

In marine Seismology, the vertical velocity, necessary to determine theNMO correction to be applied to the data to make the multiplereflections and/or the reverberations periodic or cyclic, can beactually measured by well-known techniques without the need forcomputation. For example, the vertical velocity of the seismic signal atany given location over water can be deter-' mined by timing the seismicsignal as it travels between a plate suspended below the hull of thevessel and another plate positioned at a known depth below the firstplate. Such measurements employing sonar techniques are well known tothose skilled in the art.

For seismic surveys conducted on land, the necessary vertical velocitydata is sometimes available from well surveys, for example. In anyevent, where it is necessary to compute the required velocityinformation it can be determined in the following manner.

The preferred embodiment for determining the necessary corrections tomake the multiple reflections and/or the reverberations periodic orcyclic is to use cross-correlation techniques. A method using suchtechniques for determining seismic velocity is set forth in application,Ser. No. 765,943, filed Oct. 8, 1968, and assigned to the same assigneeas this application. It is to be understood that the specification ofthe aforementioned copending application is to be a reference herein forthe purpose of setting forth a method whereby the necessary seismicvelocities may be calculated by an apparatus without the need for penciland paper, or graphical analysis, so that the aforementioned correctionsmay be determined and applied at 102.

In accordance with the disclosure of the copending application, theseismic data is collected into groups according to the various commonreflection points, and selected traces are cross-correlated over anumber of different correlation intervals. The time shifts between thecorrelated signals and the center of the correlation intervals aredetermined and used to compute the seismic velocities. The computedvelocities are compared with initially assumed velocities to retain onlythose computed velocities that are within a predetermined velocityinterval. A mean velocity is obtained from the retained velocity valueswhich may be used to provide a moveout table. The velocity intervalsused with this invention in the method of the copending application maybe determined in accordance with the results desired. For example, thefirst velocity interval may be from zero to 3,000 ft/sec. and thesuccessive velocity intervals may increase in 3,000 ft/sec. intervals.

Furthermore, in accordance with the velocity determination, brieflydescribed above, overlapping correlation intervals or time gates areapplied successively to the entire seismic signal to obtain time shiftscorresponding to the correlation intervals to obtain the time shifts forcomputing the velocity data. Where the dip is not constant, inaccordance with the present invention, the time gates used arecomparable to a few cycles of the reverberation. Where the dip isconstant, or in the case-where there are parallel reflection orrefraction boundaries, the time gates selected in accordance with thedescription of the aforementioned copending application is sufficient.

Another alternative for providing the corrections at 102 is to determinea series of time delays for various record times, and the time delayvalues could be used for correcting reverberations to a repetitivenature.

In the event that the aforementioned cross-correlations result in anambiguity as to the cycles which correlate, the cross-correlations maybegin at the end of the recorded seismic data and proceed back towardthe start of the data. At the end of a set of seismic data, there islittle time difference between the seismic signals from variousdetectors having different sourceto-detector distance, because, as iswell known, the moveout at large source-to-detector distances is small.

The deconvolution technique utilized at 103 to remove the unwantedreflections or reverberations may be any of the standard types now inuse or those briefly mentioned above. The preferred manner of removingthe time variable corrections applied at 102, after the deconvolution,is to apply the NMO for the primary reflections as previously described.

The normal processing at 105 may include filtering, automatic gaininsertion, and an NMO correction for desired primary reflections. Thedisplay at 106 may consist of only a seismic record section. However, ingeneral, normal processing at 105 would include the horizontal stackingtechnique of Mayne after the NMO is applied for the desired primaryreflection. The normal display at 106 maybe obtained from a visualmonitor, photographic seismic record sections, holographic displays, orany display technique known in the art.

This invention is not limited by the type of seismic source utilized tocreate the seismic energy and the seismic source may consist of anexplosion, a weightdrop, a sudden release of air from an air gun, or anyother source known to the seismologist. For example, a continuousvibrator having a constant frequency may be used in conjection with aholographic display. A chirp-type or FM seismic source could be usedwhere the frequency is shifted over a few octaves such as is done with aVibroseis system.

A factor, however, which must be considered with the specific type ofseismic source utilized is whether the data at 101 requiresremodification, reformatting, or editing. For example, if either of theaforementioned latter two types of seismic sources are used, the datamust be reformatted and edited prior to being corrected at 102. Suchreformatting and editing is known to those skilled in the art and formsno part of the present invention. Holographic data may be correlatedwith Fresnel zone strips or similar processes which are known. Theseismic data from a chirp-type source could be cross-correlated with thepilot trace (a recording of the chirp applied to the earth). Shouldthere be an insufficient frequency shift, or if the higher frequencieswere severely attenuated in the earth, intense ringing is observed withsuch a seismic source. The present invention is also applicable toremove ringing from the seismic data created by the seismic source.

It is to be understood that the use of the method disclosed herein doesnot eliminate the necessity for the application of many processingroutines which are now normally associated with deconvolutiontechniques. Prewhitening is still required in those instances where itis necessary prior to normal deconvolution. The requirements forprewhitening have been described by Blackman and Tukey in TheMeasurement of Power Spectra from the Point of View of CommunicationEngineering, Dover Publications, Inc., New York, 1959.

However, the particular method for the design of the inverse filter usedin the deconvolution of the data is not critical in the presentinvention. In general, if the multiple reflections or'reverberations aremade sufficiently close to a repetitive nature, almost any type ofdeconvolution may be employed.

In the preceding description, in applying the NMO correction, data froma remote detector was shifted to the time of the data from a neardetector. This simplified the description of the invention, but is not anecessary requirement. Any reference time could be used if the multiplereflections or reverberations are made repetitive. For example, in theprocessing of seismic data under certain conditions, a referenceintermediate the source and the most remote detector is preferred sothat part of the data will be shifted in one direction and part of thedata shifted in the opposite direction. This has the advantage that noneof the data will be shifted as much as when the reference is a neardetector, for example. The important concept to bear in mind is that thecorrections which are removed from the data after deconvolution must beapplied in the opposite direction or sense of the NMO corrections whichwere introduced into the data prior to the step of deconvolution. Thus,in the above instance, part of the seismic data after deconvolution willalso be shifted in one direction and the remaining portion of the siemicdata will be shifted in the opposite direction.

Frequently there is only one shallow media that causes seriousreverberations and multiple reflections. However, if there are more thanone series of reverberations, particularly from shallow media, the stepsat 102, 103, and 104 would be repeated for each series ofreverberations.

DESCRIPTION OF APPARATUS The apparatus for applying the corrections at102 and removing the corrections at 104 is illustrated in FIG. a. Thisapparatus is disclosed in US. Pat. No. 3,348,194, issued Oct. 17, 1967to William W. Witt et al. Briefly, in accordance with the descriptiontherein, data card reader 108 inserts the necessary information inmoveout accumula tor 110 via line 112. Data drum 112 contains theseismic information and also has recorded thereon a zero timeindication. Simultaneously with the transmission of the recorded data ondrum 1 14 along line 116, the reference time zero pulse is transmittedfrom drum 114 to record time counter 118 along line 120. At zero time, apulse is initiated from record time counter 118 through'rate time gate122 via line 124. Rate time gate 122 is thereby conditioned so that datacard reader 108 inserts the initial slope condition into slope register126 via line 128.

Output of slope register 126 is provided to arithmetic circuit 132 inaccordance with control pulses from a IkI-Iz oscillator via line 134.The output of arithmetic circuit 132 is inputted to moveout accumulator110 by line 136 and at every occurence of input 134, the count inmoveout accumulator 110 is updated by the count in slope register 126.After a predetermined calculation interval, rate time gate 122conditions card reader 108 to input the next information to moveoutaccumulator 110 and slope register 126, and the preceding operation isrepeated.

After the calculations have been concluded, the output from moveoutaccumulator 110 is applied to digital-to-analog converter 138 which isdriven by a chopper signal on line 140. Converter 138 then drives delayline servo 142 to position pick up head 144 in the following manner.Recording medium 146 is constantly rotated in the direction indicated.Recording head 148 is fixedly disposed opposite the recording surface ofmedium 146 to receive the uncorrected seismic data signal on line 150Pick up head 144 is rotated in either direction along the surface ofmedium 146 in accordance with the output from delay line servo 142 toprovide the necessary time correction to the seismic data from data drum114. Erase head 152 clears the medium 146 after data has been sensed bypick up head 144.

The foregoing description is applicable for the apparatus at 102,however in accordance with this invention, the data applied at 102 isremoved after the deconvolution of the data at 103. This can beaccomplished with the same apparatus merely by altering the sign of thedata within data card reader 108 so that information from slope register126 which has previously added to the count within moveout accumulator l10 is subtracted and vice-versa.

Apparatus for performing the necessary deconvolution required at 103 isillustrated in FIGS. Sb-Sd and is taken from Canadian Pat. No. 800,163,issued on Nov. 26, 1968 to Foster et al. FIGS. 5b, 5c, and 5d of thisspecification respectively conform to FIGS. 3, 4 and 5 of theaforementioned patent. The description in the Canadian Patent beginningat page 22, line '10andcontinuing to page 31, line 9 is particularlypertinent to the operation of the apparatus of the aforementionedfigures. The description therein is considered to be adequate for thepurposes of understanding the operation of the apparatus as well as forpracticing this invention, and will not be described in detail herein.

Briefly, the circuitry illustrated in block diagram format in FIG. 5bdetermines the inverse operator and the convolution of the data with theinverse operator. It is known that convolution of seismic data with aninverse operator is deconvolution as the term is used within the contextof this invention. The analog apparatus shown in FIG. 50 is capable ofimplementing a time domain inverse filter, and convolving seismic datatherewith, and the analog computer of FIG. 5d solves the necessarysimultaneous equations for determining the coefficients of the inversefilter.

Thus, in accordance with this invention, the data from pick up head 144on line 160, at 102 of FIG. 4, is applied by way of amplifier 162 torecording head 164 of magnetic belt 164 to record the data thereon fordeconvolution. As the tape moves, the recorded l 1 seismic data is movedtoward and under pickup heads 165, 166, 167, 168. The number of pick upheads is determined by the number of coefficients of the inverse filter.Each of the pick up heads is respectively connected to PM demodulators169,170,171,172, and in turn, to amplitude and polarity determiningdevices "3,174,175,176. Each of the latter devices includes a variablegain amplifier and a polarity reversing switch as described in theaforementioned patent at page 30. The inverse filter is defined byadjustment of the amplifier gain and polarity switch of the respectivedevices 173-176 thereby convolving the input signal on line 160 with theinverse filter. The signal output from devices 173-176 is recorded bysumming amplifier 178 whereupon the data may then be sent to suitableapparatus for further processing as described above.

The invention is also applicable to seismic refraction data. Furthermorereverberations may mask the desired refracted data and conventionallyapplied deconvolution deteriorates the refracted data because thesource-to-detector distances are very large. The preferred form of theinvention as applied to refraction data would comprise the followingthree steps. The introduction of a first time variable correction tomake the reverberations repetitive. Deconvolution of the corrected dataand the introduction of a second time variable shift wherein the timeshift would be equal in magnitude, but opposite in direction, to thefirst time shift correction. The time shift first introduced is an NMOdetermined for reflections based on the velocity in the reverberatingmedia, if the boundaries are parallel. If the boundaries are notparallel, the type of crosscorrelation used for reflection data isappropriate.

The equivalence between analog and digital techniques, as well as theapparatus for performing such techniques, is known to those skilled inthe art to which this invention is related. So is the equivalence alsoknown between analog and digital computers. That is, that which can becomputed by an analog computer or analog circuitry may also be computedby a suitably programmed general purpose digital computer or digitalcircuitry. Therefore, the method and apparatus of this invention maycomprise such analogous digital techniques because the modificationsnecessary to so adapt the analog techniques and circuitry describedherein are well within the ordinary skill of .one skilled in the art.

Furthermore, the invention claimed and described herein is not intendedto be carried out by an operator using paper and pencil manipulations.The utility of the invention resides entirely in the use and employmentof apparatus for automatically carrying out the invention. The use ofpaper and pencil manipulations to perform the claimed invention wouldreduce its utility solely to the performance of an academic exercisewhich would not advance the state of the art to which the inventionrelates. The necessary time and effort expended in such ever utilitysuch practice conveys.

, What is claimed is:

1. The method of processing within an automated data processing machinethe seismic data that is obtained from the detection and recordation ofseismic energy released from a seismic source to a plurality ofdetectors, wherein at least one of the detectors is remote from theenergy source, comprising the steps of:

a. introducing a first time variable shift in said seismic data from atleast one of said detectors remote from said energy source by automateddata processing means;

b. deconvolving said time shifted data within an automated dataprocessing means; and

c. introducing a second time variable shift in said deconvolved data inthe opposite direction from said first time variable shift by automateddata processing means.

2. The method of claim 1 wherein reverberations are present in saidseismic data and said time shift in step (a) is equal to the normalmoveout determined for the velocity of said reverberations in the mediathrough which said seismic energy has passed.

3. The method of claim 1 wherein reverberations are present in saidseismic data and said time shift in step (a) shifts the thereverberations so that they are periodic.

4. The method of claim 1 wherein step (b) further comprisescross-correlating within an automated data processor limited timeportions of the data from a close detector with corresponding timeportions of data from said remote detector and using the time delayobtained therefrom for the time shift.

5. The method of claim 1 wherein said second time variable shift in step(c) is equal to the difference between the normal moveout of desiredprimary reflections and said first time variable shift.

6. Apparatus for automatically processing seismic data that is obtainedfrom the detection and recordation of seismic energy released from aseismic source to a plurality of detectors, wherein at least one of thedetectors is remote from the energy source, comprising;

apparatus for introducing a time variable shift in said seismic datafrom at least one of said detectors remote from said energy source,

apparatus for deconvolving said time shifted data,

and

apparatus for introducing a second time variable shift in saiddeconvolved data in the opposite direction from said first time variableshift,.

Men

1. The method of processing within an automated data processing machinethe seismic data that is obtained from the detection and recordation ofseismic energy released from a seismic source to a plurality ofdetectors, wherein at least one of the detectors is remote from theenergy source, comprising the steps of: a. introducing a first timevariable shift in said seismic data from at least one of said detectorsremote from said energy source by automated data processing means; b.deconvolving said time shifted data within an automated data processingmeans; and c. introducing a second time variable shift in saiddeconvolved data in the opposite direction from said first time variableshift by automated data processing means.
 2. The method of claim 1wherein reverberations are present in said seismic data and said timeshift in step (a) is equal to the normal moveoUt determined for thevelocity of said reverberations in the media through which said seismicenergy has passed.
 3. The method of claim 1 wherein reverberations arepresent in said seismic data and said time shift in step (a) shifts thethe reverberations so that they are periodic.
 4. The method of claim 1wherein step (b) further comprises cross-correlating within an automateddata processor limited time portions of the data from a close detectorwith corresponding time portions of data from said remote detector andusing the time delay obtained therefrom for the time shift.
 5. Themethod of claim 1 wherein said second time variable shift in step (c) isequal to the difference between the normal moveout of desired primaryreflections and said first time variable shift.
 6. Apparatus forautomatically processing seismic data that is obtained from thedetection and recordation of seismic energy released from a seismicsource to a plurality of detectors, wherein at least one of thedetectors is remote from the energy source, comprising; apparatus forintroducing a time variable shift in said seismic data from at least oneof said detectors remote from said energy source, apparatus fordeconvolving said time shifted data, and apparatus for introducing asecond time variable shift in said deconvolved data in the oppositedirection from said first time variable shift,.